Iron and steel rust, aluminum oxidizes, and all of us wind up with a few extra "holes" by the time we are 39... In any case, replacing the tag and/or "over restoring" will bring your car into question, as wtez10 says. Ditto to Tom's request for a picture. Is the tag really "poor" from some corrosion?

Speaking from experience, as long as you can read it, leave it alone. Clean it up and preserve what you have, but do not attempt to remove, repair, or replace it. It will always be suspect. When it comes to tags and stamps---an ugly original is better than a pretty fake.